Microelectronic Engineering 86 (2009) 2305–2311

Contents lists available at ScienceDirect

Microelectronic Engineering

journal homepage: www.elsevier .com/locate /mee

Electrical and interface properties of Au/DNA/n-Si organic-on-inorganic structures

Salih Okur a,*, Fahrettin Yakuphanoglu b, Mehmet Ozsoz c, Pınar Kara Kadayifcilar c

a Department of Physics, Izmir Institute of Technology, Gulbahce Campus, Urla, Izmir 35430, Turkeyb Physics Department, Firat University, Elazig 23169, Turkeyc Department of Analytical Chemistry, Faculty of Pharmacy, Ege University, Bornova, Izmir 35100, Turkey

a r t i c l e i n f o a b s t r a c t

Article history:Received 11 July 2008Received in revised form 30 December 2008Accepted 11 April 2009Available online 22 April 2009

Keywords:Organic/inorganic junctionDNAIdeality factor

0167-9317/$ - see front matter � 2009 Elsevier B.V. Adoi:10.1016/j.mee.2009.04.017

* Corresponding author. Tel.: +90 232 750 7706; faE-mail address: salihokur@iyte.edu.tr (S. Okur).

The effect of the thickness and coverage rate of a DNA film on the electrical and interface properties of Au/DNA/n-Si organic-on-inorganic structures has been investigated. The thin film properties of the DNAdeposited on n-Si wafer were characterized by atomic force microscopy. The effect of the thicknessand coverage rate of the DNA layer was investigated by evaluating electrical parameters, such as the bar-rier height, ideality factor, series resistance, and interface state density. The thickness and coverage rateof the DNA layer significantly affects the electrical properties of the Au/DNA/n-Si organic-on-inorganicstructures. The interface state density properties of the Au/DNA/n-Si diodes were determined by conduc-tance technique. The results show that the interface state density decreases with decrease in both filmthickness and coverage rate of the DNA in an acetate buffer, modifying the electronic parameters ofthe Au/DNA/n-Si diodes.

� 2009 Elsevier B.V. All rights reserved.

1. Introduction

As a nano-device material for molecular biotechnology andnano-electronics, there are many advantages to using DNA suchas the ability to control its size through base-pair manipulationand to achieve self-assembly [1–6]. Electrostatic interactions ofDNA with positively charged surfaces are frequently consideredin the literature [7,8]. A DNA molecule (a genetic informationcarrier) is a double-stranded negatively charged polymer. DNAadsorption at negatively charged surface using self-assembledmolecular monolayer or Peptide Nucleic Acid treatments are alsoinvestigated [9]. DNA changes its conformation through environ-mental interactions with the other components of the living cells[10,11]. Several conduction mechanisms, such tunnelling (single-step super-exchange) [12], and hopping (multistep charge trans-port) [13], over guanine (G) as the DNA base with the lowestionization potential were suggested for the electrical transportthrough DNA molecules. Thus, DNA has been shown to be an insu-lator, [14–16] a semiconductor, [13,17–19] a good conductor, [6]and a proximity-induced superconductor [20].

Controlling of electronic properties of metal/inorganic diodes isthe key to fabricate reproducible metal/organic semiconductor/inorganic rectifying devices. Modifying the interface electronicstates of the metal/inorganic semiconductor structure throughDNA molecules can be essential for electronic devices for biotech-nology related applications. This could lead to an efficient barrierheight modification of the metal/semiconductor devices.

ll rights reserved.

x: +90 232 750 7707.

In several organic/inorganic Schottky diode studies [21–25],modification of the electronic parameters of the diodes by usingvarious organic materials have been demonstrated. Nicollian andBrews suggested that the interface state density properties of thediodes can be determined using conductance technique [26]. Inthis work, we have modified the electronic interface states of then-type Si Schottky diode with a DNA, and analyzed. The interfacethin film properties of the DNA in the acetate buffer layer formedon n-Si were characterized by utilizing semi-contact tapping modeatomic force microscopy (TM-AFM).

AFM has been a widely used as a powerful research tool in molec-ular biology for analyzing the structures of bio-molecules such asproteins [27–29], DNA [30–32] and others [33,34]. AFM images sam-ple surfaces by continuous scanning with a sharp tip. In simple con-tact mode AFM, a tip at the end of a flexible cantilever is used todetect sub-nanometer size changes in the interactional force as afunction of height for the molecular surface, resulting in a 2-dimen-sional plot of topographic features [35,36]. In semi-contact tappingmode atomic force microscopy (TM-AFM), the topographic image iscollected from any type of (hard or soft) surface with an oscillatingprobe tip driven near its resonance frequency by a piezoelectrictransducer that provides a driving force of a constant amplitude[37,38]. During TM-AFM scanning, an oscillating tip comes so closeto the sample surface that, it regularly touches the surface for a veryshort period of time during its oscillations. The short tip-sample con-tact time prevents any irreversible destruction on soft surfaces. Therelatively weak tip-sample interaction becomes stronger as the tipapproaches the surface and it lowers the vibrational amplitude bycreating a phase shift between measured and actuating signal [39].The AFM electronic feedback system maintains the amplitude of

2306 S. Okur et al. / Microelectronic Engineering 86 (2009) 2305–2311

tip oscillations at a user-defined set point (the operating amplitude)during construction of the surface topography. Obtaining an imageof the phase signal simultaneously with the topographical mappingof the sample surface (known as the phase contrast method) pro-vides additional information on the details of the surface structure.While the cantilever operating in the tapping mode gives topograph-ical information via Van der Waals interactions, i.e., a combination oflong-range attractive and short-range repulsive forces [40,41], thephase image allows mapping of the variation of surface propertiessuch as composition, adhesion, friction, and visco-elasticity [42,43].

In the present study, the effect of the thickness and coveragerate of the DNA on the acetate (ABS) buffer layer on the electronicinterface states of Au/DNA/n-Si organic-on-inorganic Schottkydiodes was investigated by evaluating electrical parameters suchas the barrier height (/b), ideality factor (n), series resistance (Rs),and interface state density (Dit) from conductance and capacitancemeasurements.

2. Experimental details

In this work, an n-type (N/Phos) single crystal silicon waferpre-polished on one side and having a h1 0 0i surface orientation,thickness of 0.530 mm, diameter of 100 mm and 2.00 Ohm-cmresistivity was purchased from Si-Mat Silicon Wafers Company, andused as a substrate. The Si wafer was cleaned for 10 min in boiling tri-chloroethylene, acetone, and ethanol, consecutively. Then, it wascleaned using the following series of solvent or solvent mixtures:H2SO4, H2O2, HF: H2O (1:20) solution, HNO3:HF:H2O (6:1:35), and fi-nally a HF: H2O (1:20) solution. The Si wafer was rinsed thoroughly inde-ionized water of 18 M before and after cleaning with each solventor solvent mixture. The fish sperm DNA (as lyophilized powder) wasobtained from Serva Company (Germany). The double-stranded DNA(dsDNA) stock solution (1000 mg/L) was prepared with ultra puredistilled water and kept frozen. More dilute solutions of dsDNA wereprepared in 0.05 M acetate buffer solutions, each containing 20 mMNaCl (ABS, pH 4.80). The DNA sample was diluted to 10 and 0.1 mg/L with the ABS solution. Immediately after surface cleaning, 6 ll ofthe DNA solutions at three different concentrations were coated bythe drop casting method and allowed to dry 12 h in a humid-free des-iccator. The DNA organic film thickness and homogeneity depends onits solution concentration and wetting property on the n-type Si sub-strate. Using the AFM scratch and measure method, average DNA filmthicknesses of 200 and 50 nm for DNA volumetric concentrations of10 and 0.1 mg/L, respectively were determined for the samples.

In a turbo pumped high vacuum system at a pressure of5 � 10�6 Torr, Au (99.9% purity) with a thickness of 200 nm wasthermally evaporated from a tungsten filament using a mask of2 cm diameter to form the top contact on the DNA film surfaceof the Si wafer. The back side of the Si substrate had been polishedwith high speed rotating sand paper to remove the native oxideand to create an ohmic contact before the 200 nm Au thin filmdeposition. The current–voltage (I–V) characteristics of the Au/DNA/n-Si diodes were performed with a Model 2400 KEITHLEY(Cleveland, USA) source-meter and a GPIB data transfer card forcurrent–voltage measurements. The capacitance–voltage measure-ments were measured using a Model 3532 HIOKI HITESTER LCRmeter (Bohemia, USA).

DNA samples were visualized with a Solver P47H atomic forcemicroscope (NT-MTD, Moscow, Russia) operating in the tappingmode in air at room temperature. A scanner, equipped with a scan-ning piezoelectric element (piezo scanner) with maximum scanrange of 4 lm � 4 lm � 1.5 lm was used to obtain both surfacemorphology and phase images of the DNA films that had beencoated by the drop casting method on a n-type Si substrate. Specialdiamond-like carbon (DLC) coated NSG01_DLC silicon cantilevers

(NT-MTD) with a 2 nm tip apex curvature, a length of 130 lm, aspring constant of 5.5 N/m, and a resonance frequency of 150 kHzwas used to take the topographies during the AFM scanning. Thescan resolution was 512 � 512 pixels. The images were processedby the linear flattening method in order to remove the backgroundslope. The Nova 914 software package was used for controlling theSPM system and analyzing the AFM images.

3. Results and discussion

3.1. AFM results of the DNA thin films deposited on Si substrate

In this work, the AFM tapping mode phase imaging (TM-AFMPI)was used to distinguish the DNA molecular film layer from the ABSbuffer. When imaging DNA molecules by AFM on any smooth sur-face, the DNA concentration has to be prepared so low that theindividual DNAs can be found easily on the scanned surface sepa-rated from each other. In our case, two different concentrationshave been prepared to investigate film formation properties ofthe DNA in ABS buffer on n-type Si wafer surface, rather than visu-alizing the individual DNA molecules. Nevertheless around theedge of the drop casted DNA film, the individual DNA moleculescould be seen separately. We have experienced that the DNA cannot be imaged unless it is dried following the adsorption process.It aggressively interacts with the tip and changes the tip resonancefrequency by loading extra mass as a result of sticking to the tip.But a few DNA molecules suspended on the tip enhances the inter-action between the surface and the tip. The resolution of bothtopography and especially the phase image increases dramaticallyafter a few scans on the DNA covered film surface.

The AFM topography and AFM phase image of the 200 nm DNAfilm with the volume concentration of 10 mg/L on the ABS buffersurface are given in Fig. 1a and b, respectively. The color bars onthe right sides show surface height and phase angle changes,respectively. The lighter regions have higher altitude on the 2Dsurface. On the other hand, the lighter regions on the phase imageshow that more repulsive interaction takes place between the tipand the DNA surface. The topography shows a smooth and com-pletely covered DNA film surface, while the film disorders and pin-holes of the DNA film can be distinguished from the contrast of theAFM phase image clearly. The DNA film thickness on ABS bufferlayer was measured around 3 nm by taking a cross sectional heightprofile from one of the pinholes on the film surface as shown inFig. 1c. The coverage rate of DNA film was obtained as 96% overthe area of 3 lm � 3 lm of the film from the statistical calculationsof the phase image as given in Fig. 1d.

Fig. 2a and b show the AFM topography and AFM phase imageof the 50 nm DNA film with the volume concentration of 0.1 mg/L, respectively. The film disorders and coverage rate is more pro-nounced in both the topography and phase image. The lighter re-gion represents the DNA film, while the darker region representsthe underlying ABS buffer layer. The DNA film thickness on ABSbuffer layer has been measured around 4 nm by taking a heightprofile across a line through edge of a pinhole on the DNA film sur-face as shown in Fig. 2c. In Fig. 2d, the statistical phase angle-fre-quency profile of the phase image shows 60% darker regions(buffer layer). The remaining 40% coverage can be deduced forthe DNA film coverage over the area of 3 lm � 3 lm of the driedDNA film concentration of 10 and 0.1 mg/L.

3.2. The current–voltage characteristics of Au/DNA/n-Si diodes

Fig. 3 shows the current–voltage characteristics of the Au/DNA/n-Si diodes. The diodes show a rectifying behavior. The rectifyingproperty of the Au/DNA/n-Si diode is improved with film thickness.

Fig. 1. (a) AFM topography of DNA, (b) AFM phase image of the 200 nm DNA film with the volume concentration of 10 mg/L in the ABS buffer layer on n-Si surface, (c) thecross-sectional profile of DNA taken from a pinhole on the film, and (d) the coverage rate of DNA is obtained as 96% over a scan area of 3 lm � 3 lm of the film.

Fig. 2. (a) AFM topography of DNA, (b) AFM phase image of the 50 nm DNA film with the volume concentration of 0.1 mg/L in the ABS buffer layer on n-Si surface, (c) thecross-sectional DNA profile taken from edge of the film, and (d) the coverage rate of DNA is obtained as 40% over a scan area of 3 lm � 3 lm of the film.

S. Okur et al. / Microelectronic Engineering 86 (2009) 2305–2311 2307

The current–voltage characteristics of the diodes can be analyzedby the following relation [44],

I ¼ Io expq V � IRsð Þ

nkT

� �1� exp � qðV � IRsÞ

kT

� �� �ð1Þ

where n is the ideality factor, k is the Boltzmann constant, Rs isthe series resistance, V is the applied voltage, T is thetemperature and Io is the reverse saturation current. The idealityfactors of the diodes were determined from the I–V characteris-tics. The ideality factors are higher than unity due to the DNAfilm with ABS layer and possible thin oxide layer and series

resistance at the interface. The oxide layer at the interfacemay be formed during surface preparation of the Au evapora-tion. The presence of interfacial layer, series resistance effectand bulk resistance of the DNA film affects the electronicparameters of the organic–inorganic structures. In order to ana-lyze the series resistance effect, we have used Cheung’s methodto determine the diode parameters with the Cheung’s functionsexpressed as [45],

dVd lnðIÞ ¼ n

kTqþ IRs ð2Þ

Voltage (V) Voltage (V)

3x10-7

3x10-6

3x10-5

3x10-4

3x10-3

3x10-2

Cur

rent

(A)

-2.1 -1.6 -1.1 -0.6 -0.1 0.4 0.9 1.4 1.9 -2.1 -1.6 -1.1 -0.6 -0.1 0.4 0.9 1.4 1.9

2x10-7

2x10-6

2x10-5

2x10-4

2x10-3

2x10-2

Cur

rent

(A)

(a) (b)

Fig. 3. Current–voltage characteristics of the Au/DNA/n-Si diodes (a) for 200 nm film layer, and (b) for 50 nm film layer.

2308 S. Okur et al. / Microelectronic Engineering 86 (2009) 2305–2311

and

HðIÞ ¼ V � nkTq

lnIo

AA�T2

� �¼ IRs þ n/b ð3Þ

where Io is the saturation current, A is the contact area, A* is theRichardson constant (112 A cm�2 K�2 for n-Si) [46], Rs is the seriesresistance and /b is the barrier height. The plots of dV/dlnI vs. Iand H(I) vs. I of the diodes are shown in Fig. 4a and b and theobtained parameters are given in Table 1. The Rs and n values werecalculated from the slope and intercept of the dV/dlnI vs. I plot andthe results are given in Table 1. The series resistance is decreasedwith decreasing film thickness. This suggests that the low value ofseries resistance obtained for the diodes with thinner DNA layergives a Schottky diode with better performance. The ideality factorn and barrier height /b values are higher for the thicker DNA layer(200 nm). This indicates that organic film thickness affects the ide-ality factor and barrier height. Furthermore, at intermediate andhigher voltages, the organic layer may change the charge transportmechanism of the diode. Thus, to be sure about this, I–V character-istics of the diodes were plotted in logarithmic scale, as shown inFig. 5. The logarithmic I–V curves in the figures indicate three differ-ent current regions exhibiting the power law behavior of I � Vm,where m shows the slope of each region. The m values for regionsI, II and region III of the Au/DNA (200 nm)/n-Si and Au/DNA(50 nm)/n-Si were found to be 1.08, 8.91, 3.01 and 1.34, 10.48,2.03, respectively. The obtained m values correspond to ohmic,trapped charge-limited current (TCLC), and space charge-limitedcurrent (SCLC) mechanisms. The third region for the diodes is trans-formed from TCLC to SCLC mechanism with decrease in DNA filmthickness. In the TCLC mechanism, the current is controlled byexponential distribution of traps in the band gap of the DNA layer.The obtained results reveal that the film thickness and coveragerate of the DNA layer modifies the I–V characteristics of the diodeas a result of various charge related conduction mechanisms inthe Au/DNA/n-Si diode.

3.3. Interface state density properties of the Au/DNA/n-Si diodes

The plots of capacitance vs. frequency under bias voltages of 0and 0.5 V are shown in Fig. 6. The capacitance decreases withincreasing frequency and tends to non-dispersive. At lower frequen-

cies, the capacitance increases with decreasing frequency. This is anindication of presence of a continuous distribution of the interfacestates. The non-dispersive behavior of capacitance suggests thatthe interface states in equilibrium with the semiconductor do notcontribute to the capacitance. In the non-dispersive region, thecharges at the interface states cannot follow the fast alternating cur-rent signal. The capacitance at lower frequencies corresponds to thesum of space-charge capacitance and interface capacitance, while athigher frequencies the total capacitance arises mostly from thespace-charge capacitance [43].

For the conductance model, the parallel capacitance Cp and con-ductance Gp relations are expressed as [45],

Cp ¼ Cs þCit

1þ ðxsitÞ2ð4Þ

and

Gp

x¼ qxsitDit

1þ ðxsitÞ2ð5Þ

where Cit = qDit, sit = RitCit and sit is the interface trap constant. Thenormalized conductance is expressed as,

Gp

x¼ qADitsit

2xsitlnð1þx2s2

itÞ ð6Þ

where Dit is the density of the interface states, q is the charge of theelectron, x is the angular frequency, s is the time constant of theinterface states. The parallel conductance between measured con-ductance and capacitance is expressed as following,

Gp

x¼ xGmC2

ox

G2m þx2ðCox � CmÞ2

ð7Þ

where Cox is the capacitance of oxide layer, Gm and Cm are measuredconductance and capacitance respectively. The interface state den-sity is determined using the following relation,

Dit ¼ðG=xÞmax

0:402qA: ð8Þ

Fig. 7 shows the plots of (G/x) vs. f of the diode at different biasvoltages. The plots indicate a peak due to the presence of interfacestates. The shift in the peak position to higher frequencies with

1.23

1.27

1.31

1.35

1.39

1.43

H(I)

(V)

0x100 10-3 2x10-3 3x10-3 4x10-3

I(A)

0.00

0.05

0.10

0.15

0.20

dV/d

lnI (

V)

(a)

10-2 2x10-2 3x10-2 4x10-2 5x10-2 6x10-2 7x10-2

I(A)

0.00

0.10

0.20

0.30

0.40

0.50

0.60

0.70

0.80

0.90

dV/d

lnI (

V)

1.10

1.20

1.30

1.40

1.50

1.60

1.70

1.80

1.90

H(I)

(V)

(b)

Fig. 4. Plots of dV/dln(I) vs. I and H(I) vs. I of the Au/DNA/n-Si diodes (a) for 200 nm film layer, and (b) for 50 nm film layer.

Table 1Electronic parameters of the diodes.

Diode n C�V /b (eV) I�V /b (eV) dV=dlnIRs (X) HðIÞRs (X) Dit (eV cm�2)

Au/DNA(200 nm)/n-Si 1.41 1.35 0.87 41.84 42.51 8.56 � 1012

Au/DNA(50 nm)/n-Si 1.35 1.17 0.75 13.23 11.19 2.54 � 1012

0.0 0.1 1.0Voltage (V) Voltage (V)

3x10-8

3x10-7

3x10-6

3x10-5

3x10-4

3x10-3

3x10-2

Cur

rent

(A)

region I

region II

region III

region I

region II

0.0 0.1 1.02x10-8

2x10-7

2x10-6

2x10-5

2x10-4

2x10-3

2x10-2

2x10-1

Cur

rent

(A)

region III(a) (b)

Fig. 5. Current–voltage characteristics in logarithmic scale of the Au/DNA/n-Si diodes (a) for 200 nm film layer, and (b) for 50 nm film layer.

S. Okur et al. / Microelectronic Engineering 86 (2009) 2305–2311 2309

Fig. 6. Plots of C–f of the Au/DNA/n-Si diodes.

Fig. 7. Plots of G/x–f of the Au/DNA/n-Si diodes at bias various voltages.

Au/DNA(50 nm)/n-Si

-2.0 -1.5 -1.0 -0.5 0.0 0.5

Voltage (V)

0

2

4

6

8

C-2 (F

-2)x

1018 Au/DNA(200 nm)/n-Si

Fig. 8. Plot of C�2–V of the Au/DNA/n-Si diodes.

2310 S. Okur et al. / Microelectronic Engineering 86 (2009) 2305–2311

increasing bias voltage suggests that the time constant of interfacestates is decreased with applied voltage. The calculated density ofinterface state (Dit) of the Au/DNA/n-Si diodes is given in Table 1.The decrease in the Dit value of the Au/DNA/n-Si diode withdecreasing film thickness suggests that the electronic parametersare improved with decreasing interface state density as a result oflower coverage rate of DNA molecules in the interface.

3.4. Capacitance–voltage characteristics of the Au/DNA/n-Si diodes

Fig. 8 shows the capacitance–voltage (C–V) curves of the Au/DNA/n-Si diodes at 100 kHz. The C–V curves of the diodes wereplotted in the form of C�2–V and are given in Fig. 8. The C�2 vs. Vplot of the Au/DNA (50 nm)/n-Si diode gives a shorter flat region,whereas for the diode having thicker DNA thickness gives a longerflat region. This shows that the uniformity of the interface of thediode is improved with the thicker film thickness. The capaci-tance–voltage characteristics of the diode can be analyzed by thefollowing relation [47],

1C2 ¼

2ðVbi þ VÞA2esqNd

ð9Þ

where Vbi is the built-in potential, es is the dielectric constant ofsemiconductor (11.7 for Si) [47] and Nd is the donor concentration.The barrier height /b can be obtained by the following relation,

q/BðC�VÞ ¼ qVbi þ kT lnNc

Nd

� �ð10Þ

where Nc is the effective density of state in the conduction band ofsilicon (Nv = 2.8 � 1019 cm�3).

Nd and Vbi values for the diodes were calculated from the slopeand intercept of C�2–V plot. The barrier height /b values of thediodes were calculated using the obtained Nd and Vbi values. The/b value of the diode for the thin DNA layer (50 nm) is low in com-parison to that of the thicker DNA layer (200 nm). This indicatesthat the film thickness reduces the barrier height of the n-Si/DNA/Au diodes. The /b(C–V) value is obtained higher than the/b(I–V) value due to the barrier inhomogeneities of the interfaciallayer thickness and distributions of the interfacial charges[48,49]. Furthermore, this also shows the I–V and C–V characteris-tics have different nature. Thus, the barrier heights obtained fromI–V to C–V measurements are not the same [50].

4. Conclusions

The interface structure and electrical properties of Au/DNA/n-Siorganic-on-inorganic structure have been investigated by atomicforce microscopy, current–voltage and capacitance–voltage meth-ods. The thickness and coverage rate of the DNA layer significantlyaffect electrical parameters such as the, barrier height (/b), idealityfactor (n), series resistance (Rs) and interface state density (Dit) ofthe Au/DNA/n-Si organic-on-inorganic structures. The interfacestate density for the Au/DNA/n-Si diodes was found to be �1012

(eV cm�2) and interface state density decreases with the decreasein film thickness. The low value of series resistance and interfacestate density gives the high performance of the Au/DNA/n-Si or-ganic-on-inorganic diode for thin DNA layer thickness.

Acknowledgements

This study was financially supported by Turkish State PlanningOrganization (DPT) with the research project numberDPT2003K120390. Dr. Ritchie Eanes is acknowledged for his tech-nical and grammatical proof reading suggestions.

S. Okur et al. / Microelectronic Engineering 86 (2009) 2305–2311 2311

References

[1] C.A. Mirkin, R.L. Letsinger, R.C. Mucic, J.J. Storhoff, Nature (London) 382 (1996)607.

[2] S.J. Tans, A.R.M. Vershueren, C. Dekker, Nature (London) 393 (1998) 49.[3] C.J. Murphy, M.R. Arkin, Y. Jenkins, N.D. Ghatlia, S.H. Bossmann, N.J. Turro, J.K.

Barton, Science 262 (1993) 1025.[4] Y. Okahata, T. Kobayashi, K. Tanaka, M. Shimomura, J. Am. Chem. Soc. 120

(1998) 6165.[5] Y. Maeda, H. Tabata, T. Kawai, Appl. Phys. Lett. 79 (2001) 1181.[6] M.A. Reed, C. Zhou, C.J. Muller, T.P. Burgin, J.M. Tour, Science 278 (1997)

141.[7] C.R. Clemmer, T.P. Beebe, Scann. Microsc. 6 (1992) 319.[8] H.G. Hansma, I. Revenko, K. Kim, D.E. Laney, Nucl. Acids Res. 24 (1996) 713.[9] M. Fojta, V. Vetterl, M. Tomschik, F. Jelen, P. Nielsen, J. Wang, E. Palecek,

Biophys. J. 72 (1997) 2285.[10] S. Takeishi, J. Chem. Phys. 120 (2004) 550.[11] S.M. Parsons, J. Struct. Biol. 119 (1997) 99.[12] B. Giese, J. Amaudrut, A.K. Kohler, M. Spormann, S. Wessely, Nature 412 (2001)

318.[13] D. Porath, A. Bezryadin, S. de Vries, C. Dekker, Nature 403 (2000) 635.[14] E. Braun, Y. Eichen, U. Sivan, G. Ben-Yoseph, Nature 391 (1998) 775.[15] P.J. de Pablo, F. Moreno-Herrero, J. Colchero, J.G. Herrero, P. Herrero, A.M. Baro,

P. Ordejon, J.M. Soler, E. Artacho, Phys. Rev. Lett. 85 (2000) 4992.[16] Y. Zhang, R.H. Austin, J. Kraeft, E.C. Cox, NP. Ong, Phys. Rev. Lett. 89 (2002)

198102.[17] K.H. Yoo, D.H. Ha, J.O. Lee, J.W. Park, J. Kim, JJ. Kim, H.Y. Lee, T. Kawai, H.Y. Choi,

Phys. Rev. Lett. 87 (2001) 198102.[18] J.S. Hwang, K.J. Kong, A. Ahn, G.S. Lee, D.J. Ahn, S.W. Hwang, Appl. Phys. Lett. 81

(2002) 1134.[19] B. Hartzell, B. McCord, D. Asare, H. Chen, J.J. Heremans, V. Soghomonian, Appl.

Phys. Lett. 82 (2000) 4800.[20] J.R. Heath, P.J. Kuekes, G.S. Snider, R.S. Williams, Science 280 (1998) 1716.[21] I.H. Campbell, S. Rubin, T.A. Zawodzinski, J.D. Kress, R.L. Martin, D.L. Smith,

Phys. Rev. B 54 (1996) 14321.[22] T. Kılıçoglu, Thin Solid Films 516 (2008) 967.[23] F. Yakuphanoglu, Sensors Actuat. A 141 (2008) 383.[24] S�. Aydogan, M. Saglam, A. Türüt, Microelectr. Eng. 85 (2008) 278.

[25] M.E. Aydin, F. Yakuphanoglu, T. Kılıçoglu, Synth. Met. 157 (2007) 1080.[26] E.H. Nicollian, J.R. Brews, MOS (Metal Oxide Semiconductor) Physics and

Technology, Wiley, New York, 1982.[27] A. Brisson, W. Bergsma-Schutter, F. Oling, O. Lambert, I. Reviakine, J. Cryst.

Growth 196 (1999) 456.[28] I. Reviakine, A. Simon, A. Brisson, Langmuir 16 (2000) 1473–1477.[29] M.E. Gracheva, A.L. Xiong, A. Aksimentiev, K. Schulten, G. Timp, J.P. Leburton,

Nanotechnology 17 (2006) 622.[30] J.B. Heng, C. Ho, T. Kim, R. Timp, A. Aksimentiev, Y.V. Grinkova, S. Sligar, K.

Schulten, G. Timp, Biophys. J. 87 (2004) 2905.[31] J. Lagerqvist, M. Zwolak, M. Di Ventra, Nano Lett. 6 (2006) 779.[32] J. Vesenka, M. Guthold, C.L. Tang, D. Keller, E. Delaine, C. Bustamante,

Ultramicroscopy 42–44 (1992) 1243–1249.[33] I.A. Mastrangelo, M. Bezanilla, P.K. Hansma, P.V.C. Hough, H.G. Hansma,

Biophys. J. 68 (1994) 293.[34] Y. Fang, J. Yang, J. Phys. Chem. Ser. B 101 (1997) 441.[35] G. Binnig, C. Gerber, E. Stoll, R.T. Albrecht, C.F. Quate, Europhys. Lett. 3 (1987)

1281.[36] G. Schitter, R. Stark, W. Stemmer, Ultramicroscopy 100A (2004) 253.[37] H.G. Hansma, J. Hoh, Annu. ReV. Biophys. Biomol. Struct. 23 (1994) 115.[38] M. Luna, J. Colchero, A.M. Baro, Appl. Phys. Lett. 72 (1998) 3461.[39] F. Moreno-Herrero, P.J. De Pablo, J. Colchero, J. Gomez-Herrero, A.M. Baro, Surf.

Sci. 453 (2000) 152.[40] J. Kokavecz, O. Marti, P. Heszler, A. Mechler, Phys. Rev. B 73 (2006) 15.[41] G.R. Jayanth, Y. Jeong, C.H. Menq, Rev. Sci. Instrum. 77 (2006) 5.[42] S.N. Magonov, V. Elings, V.S. Papkov, Polymer 38 (1996) 297.[43] H.G. Hansma, K.J. Kim, D.E. Laney, R.A. Garcia, M. Argaman, S.M. Parsons, J.

Struct. Biol. 119 (1997) 99.[44] S.K. Cheung, N.W. Cheung, Appl. Phys. Lett. 49 (1986) 85.[45] S�. Karatas�, A. Türüt, Vacuum 74 (2004) 45–53.[46] E.H. Rhoderick, R.H. Williams, Metal-Semiconductor Contacts, second ed.,

1988, p. 411.[47] S.M. Sze, Physics of Semiconductor Devices, second ed., John Wiley & Sons Inc.,

Wiley, NewYork, 1981. p. 431.[48] E.H. Nicollian, A. Goetzberger, Appl. Phys. Lett. 7 (1965) 216.[49] Y.P. Song, R.L. Van Meirhaeghe, W.H. Laflere, F. Cardon, Solid State Electron. 29

(6) (1986) 633.[50] Ö. Güllü, Ö. Barıs�, M. Biber, A. Türüt, Appl. Surf. Sci. 254 (2008) 3039.